BANGKOK (AP) — Thai authorities are cracking down on vendors in Bangkok’s main backpacker district selling cheap doses of balloons filled with nitrous oxide, or laughing gas, to tourists.

Eight vendors have been arrested since police first noticed the trend a few weeks ago along Khao San Road, a pedestrian street lined with bars and cheap hotels, police Lt. Col. Pitiphan Kridakorn said Monday.

Each dose sells for about 130 baht, or $4.

Pitiphan said the sale of drug-filled balloons started in popular beaches, such as Koh Samui, Pattaya and Koh Pagnan — an island known for “full moon” parties fueled by drugs and alcohol.

He said police are trying to curb sales of the drug-filled balloons in the capital before it gains wider popularity.

Illicit sales of nitrous oxide could cause harmful side effects to users and give the country a negative image, he said.

Those arrested face up to five years in prison and a $300 fine for selling medicine without authorization.